[New Multicolumn] Make layers paint properly in columns.
https://bugs.webkit.org/show_bug.cgi?id=110296.

Reviewed by Sam Weinig.

Source/WebCore: 

Construct the style for the RenderMultiColumnFlowThread such that it
establishes a stacking context (by being position:relative
with a z-index of 0). This causes the layer collection code
to correctly ignore the RenderFlowThread layers during normal
painting.
        
Fix more clipping bugs to eliminate subpixel layout differences
between old and new multicolumn.

Test: fast/multicol/newmulticol/layers-in-multicol.html

* rendering/RenderMultiColumnBlock.cpp:
(WebCore::createMultiColumnFlowThreadStyle):
(WebCore):
(WebCore::RenderMultiColumnBlock::addChild):
Use a custom style (similar to what normal RenderFlowThreads do)
for the RenderMultiColumnFlowThread that makes it a stacking
context.
        
* rendering/RenderMultiColumnSet.cpp:
(WebCore::RenderMultiColumnSet::flowThreadPortionOverflowRect):
Fix the overflow calculations out the leftmost and rightmost
columns to actually look at the physical placement of the columns.
The old code was backwards for RTL.
        
* rendering/RenderRegion.cpp:
(WebCore::RenderRegion::overflowRectForFlowThreadPortion):
Fix the clipping done by overflowRectForFlowThreadPortion to min
and max with the passed-in portion rect so that subclasses that
inflate the rect prior to passing it into the method don't have that
inflation discarded.

LayoutTests: 

* fast/multicol/newmulticol/layers-in-multicol-expected.html: Added.
* fast/multicol/newmulticol/layers-in-multicol.html: Added.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@143467 268f45cc-cd09-0410-ab3c-d52691b4dbfc